1. Field of the Invention
The present invention relates to an injector drive circuit used in an automobile fuel injection device and the like.
2. Description of the Related Art
The practical application of a direct injection type gasoline engine that directly injects fuel into a cylinder of an automobile engine in which an injector drive circuit is used, is proceeding. The direct injection type gasoline engine has problems of a reduction in exhaust emission due to lean-burn and a reduction in fuel consumption rate in particular.
With this background, the driving of an injection valve needs to make faster the response time of the injection valve to an injection signal and control the injection valve proportionally from a range in which the time width of the injection signal is small. As means therefor, there has generally been known a method for applying a high voltage to the injection valve on the rising edge of the injection signal to cause a large current to flow therethrough thereby to shorten a valve open time, and thereafter controlling a holding current for holding the valve open.
Such a boost convertor as described in, for example, JP-2002-61534-A is required to generate a high voltage. One example of the performance of this boost convertor will be shown. That is, the boost convertor boosts a battery voltage from a battery voltage (14[V]) to 65[V] or so and supplies a peak current of 10 [A] or so. Further, when the maximum speed is taken as 6600 [rpm], for instance with a six-cylinder engine, the high voltage drives an injection valve for each time 3 [ms]. It is therefore necessary that the high voltage is returned to a predetermined value during 3 [ms] after the injection valve has been driven once. Further, the boost convertor assumes such specifications as to be capable of assuring the battery voltage up to 10[V].
Further, attention has been given to a technology called a multiple fuel injection for the purpose of low fuel consumption and a reduction in exhaust emission in the direct injection type gasoline engine. Multiple fuel injection means that fuel injected at a time relative to one stroke of a conventional piston is injected in several batches. The multiple fuel injection enhances combustion efficiency of gasoline and enables a reduction in NOx and the like.